Stage and supporting mechanism for supporting movable mirror on stage

ABSTRACT

A movable mirror supporting mechanism for supporting a movable mirror forming a part of an interferometric measurement system and having a rectangular cross-section onto an object, with a reflecting surface of said movable mirror being arranged normal to a measuring direction. The movable mirror supporting mechanism includes at least two fixtures for securing the movable mirror from above onto respective movable mirror supporting portions provided on the object. Positions on the movable mirror at which the movable mirror is secured onto the measuring object by the fixtures are defined to be out of that area of the reflecting surface of the movable mirror which is utilized for interferometric measurement.

BACKGROUND OF THE INVENTION

The present invention relates to a stage having a movable mirrorattached thereto, for carrying a material piece such as a reticle or awafer and used in a projection exposure apparatus, for example, and alsorelates to a supporting mechanism for supporting such a movable mirroron such a stage. More particularly, the present invention relates to astage having a movable mirror attached there to, the movable mirrorforming a part of an interferometric measurement system for measuringthe position of the stage, and also relates to a supporting mechanismfor supporting such movable mirror on such a stage.

In a lithographic process used to fabricate semiconductor devices or thelike, there have been commonly used demagnification projection exposureapparatus of the type in which a wafer undergoes stepping movements forsequential exposure operations to a plurality of exposure sites on thewafer (the exposure apparatus of this type are called "steppers"). Theapparatus has a moving stage for carrying a material piece such as awafer and capable of two-axis translational movements in a plane (or"XY-stage"). Further, in order to measure X- and Y-coordinate positionsof the moving stage, a pair of light wave interferometric measurementsystems are commonly used as an apparatus for measuring the coordinatepositions (distances) by utilizing the interference between two coherentlight beams such as laser beams.

Generally, the apparatus of this type comprises a reflecting mirror (ormovable mirror) which is mounted on one end of the moving stage (whichis the measuring object) such that it extends along one side edge of themoving stage and in a direction normal to the measuring direction. Theapparatus further comprises an interferometer which is disposed outsidethe moving stage so as to face the mirror surface of the movable mirror.The interferometer emits a beam of frequency-stabilized laser (such ashelium-neon laser) to the movable mirror so as to provide interferencebetween the beam reflected by the movable mirror and another beam notreflected by the movable mirror, which interference is used to achievevery precise measurement with a high resolution of about 0.01 μm, forexample. The interferometer cannot operate without the moving mirrorprovided on the measuring object (or moving stage), and so the movablemirror forms part of the interferometric measurement system.

FIG. 1 schematically shows an exemplified supporting mechanism forsupporting a movable mirror on a moving stage, which may be used for theapparatus of this type. As is common in the art, a reflecting mirror ormovable mirror 1 (shown by imaginary lines in FIG. 1) is formed by aglass body having a rectangular cross-section with four side surfaces,one of which is silvered to form a mirror surface. The movable mirror 1is supported in the vertical direction by a pair of movable mirrorsupporting surfaces 4A and 4B, which comprise top surfaces of respectiveraised portions formed on a top surface of an elongate, belt-like raisedportion 3 having a predetermined width. The raised portion 3 is formedon one side edge of the stage body 2, and the movable mirror supportingsurfaces 4A and 4B are spaced apart from each other in the longitudinaldirection of the raised portion 3. The movable mirror 1 is furthersupported in the measuring direction (represented by arrow A in FIG. 1)such that it is clamped between i) a pair of supporting surfaces 6A and6B and ii) an associated pair of preloaded push pins 9A and 9B. Thesupporting surfaces 6A and 6B are vertical, rectangular surfaces on theinside of corresponding pair of reference protrusions 5A and 5B having abox-like shape and formed on the top surface of the stage body 2 atpositions adjacent to the movable mirror supporting surfaces 4A and 4B,respectively, and on one side of these surfaces 4A and 4B in themeasuring direction. The push pins 9A and 9B are slidably supported bycorresponding pair of push-pin-supporting protrusions 7A and 7B formedon the top surface of the stage body 2 at positions adjacent to themovable mirror supporting surfaces 4A and 4B, respectively, and on theother side of these surfaces 4A and 4B in the measuring direction. Thepush pins 9A and 9B are inserted from the outside of thepush-pin-supporting protrusions 7A and 7B into corresponding holesformed in the protrusions 7A and 7B, respectively, and are urged byrespective, associated coil springs 8A and 8B toward the supportingsurfaces 6A and 6B, respectively. The push pins 9A and 9B have flat tipend surfaces.

Unfortunately, this conventional movable mirror supporting mechanismsuffers from a problem that when the stage body 2 supporting thereflecting mirror or movable mirror 1 is driven in a certain direction(e.g. in the measuring direction), an inertial force produced by theacceleration of the driven stage body 2 acts on the moving mirror 1which is clamped between i) the supporting surfaces 6A and 6B of thereference protrusions 5A and 5B and ii) the preloaded push pins 9A and9B facing to the supporting surfaces 6A and 6B, resulting in that thecoil springs 8A and 8B may possibly yield or be compressed to allow themovable mirror 1 to displace apart from the supporting surfaces 6A and6B into an erroneous position and, in certain circumstances, even remainin that erroneous position. This results in that the position(translational or angular) of the movable mirror 1 relative to the stagebody 2 may possibly change while the stage body 2 is being driven, andthat a considerable error may possibly occur in the measured valueprovided by the interferometric measurement system.

In view of the foregoing, it could be one approach to this problem that,instead of clamping the moving mirror 1 by urging it against thesupporting surfaces 6A and 6B by the preloaded push pins 9A and 9B, themoving mirror 1 is fixedly secured onto the stage body 2, so that anydisplacement of the movable mirror 1 relative to the stage body 2 may beprevented even when an inertial force produced by the acceleration ofthe driven stage body 2 acts on the movable mirror 1.

However, as commonly known to those skilled in the art, the glass bodyconstituting the movable mirror 1 cannot be rigid enough to beconsidered as an ideal rigid body; in fact, various forces acting onsuch movable mirror 1 would cause minute but nonnegligible deformationsof the moving mirror 1. In addition, any deformation of the mirrorsurface of the movable mirror 1 which may be caused by the supportingarrangement may greatly affect the measuring accuracy of theinterferometric measurement system because the resolution of themeasurement is very high (about 0.01μμ). As the result, the arrangementin which a movable mirror is fixedly secured onto a stage body by meansof fixtures has not been used so far.

Regarding the materials for a stage body or material piece support and amovable mirror or reflecting mirror for use with an interferometer andsupported on the stage body, there have been three major options. Thefirst option is that the movable mirror is made of an optical glasswhile the stage body is made of a ceramic material. The second option isthat either of them is made of an optical glass. The third option isthat both of them are made of a ceramic material.

Unfortunately, with a conventional stage or a material piece supportcomprising a movable mirror made of an optical glass and a stage bodymade of a ceramic material, the movable mirror may be subject todeformation caused by a temperature change, since the movable mirror andthe stage body are made of different materials so that they havedifferent thermal expansion coefficients. That is, when the temperaturein the environment of the movable mirror changes, the movable mirror andthe stage body produce different thermal expansions, resulting in someharmful deformation of the mirror surface of the movable mirror, whichleads to a deterioration in the accuracy of the measured positions ofthe stage body determined by the interferometer.

In the case where the movable mirror and the stage body are made of anoptical glass or optical glasses, the stage body should have a lessrigidity and necessitate a higher cost than a stage body made of aceramic material. The less rigidity of the stage body results in theless stability in positioning of the mirror surface of the movablemirror carried on the stage body, leading to a possible deterioration ofthe measuring accuracy of the interferometer.

In the case where the movable mirror and the stage body are made of aceramic material, there arise a problem that the reflectivity of themovable mirror is lowered by the existence of pores in the ceramicmaterial. In general, powder which is to be sintered into a certainceramic material has a porosity of about 40%. Through the sinteringprocess, the porosity lowers due to various factors including thesurface tension of molten particles. Thus, most of the spacecontributing the porosity vanishes during the sintering process, and alittle space remains after the sintering process and forms pores in theceramic material. Apparently, a higher porosity of the ceramic materialfor the movable mirror results in a lower reflectivity of the movablemirror. The porosity of a ceramic material may be reduced by suitablyselecting the powder and sintering method used for the ceramic material,and thereby a more compact ceramic material may be obtained; however, ingeneral, a highly compact ceramic material created in this manner isvery costly. Accordingly, when the stage body and the movable mirrorwere formed as an integral part made of a certain, low-porosity, compactceramic material, there would arise a problem that a large amount ofsuch costly ceramic material is required, which increases the total costof the stage.

SUMMARY OF THE INVENTION

It is a primary object of the present invention to provide a supportingmechanism for supporting a reflecting mirror or movable mirror on anobject to be measured, in which any displacement of the movable mirrorrelative to the object may be effectively prevented, which couldotherwise be caused by the inertial force due to the movement of themeasuring object.

It is another object of the present invention to provide a movablemirror supporting mechanism, in which any harmful deformation of thereflecting surface of the movable mirror may be effectively suppressed.

It is a further object of the present invention to provide a stagehaving a material piece support or stage body which is inexpensive andhas a sufficient rigidity.

It is a still further object of the present invention to provide a stagehaving a stage body and a movable mirror or reflecting mirror supportedon the stage body, in which the stage body has a sufficient rigiditywhile the movable mirror has a high reflectivity.

According to one aspect of the present invention, there is provided amovable mirror supporting mechanism for supporting a movable mirrorforming a part of an interferometric measurement system and having arectangular cross-section onto an object to be measured with areflecting surface of the movable mirror being arranged normal to ameasuring direction, the movable mirror supporting mechanism including:at least two fixtures for securing the movable mirror from above ontorespective movable mirror supporting portions provided on the object;and positions on the movable mirror at which the movable mirror issecured onto the object by the fixtures being defined as being out ofthat area of the reflecting surface of the movable mirror which isutilized for interferometric measurement.

With this movable mirror supporting mechanism, it is important that themovable mirror is secured from above onto movable mirror supportingsurfaces of the object by means of at least two fixtures, and thosepositions on the movable mirror at which the movable mirror is securedis defined as being out of that area of the reflecting surface of themovable mirror which is utilized for interferometric measurement. Byvirtue of this, any displacement of the movable mirror relative to theobject may be prevented, which displacement could be otherwise caused bythe movement of the object. Further, by virtue of the above, anydeformation of the movable mirror will be confined to those areas of themeasuring surface (or the reflecting surface) of the movable mirrorwhich are out of the area to be utilized for interferometricmeasurement, resulting in that such deformation of the movable mirrormay be prevented from affecting the measurements provided by theinterferometer.

According to another aspect of the present invention, there is provideda movable mirror supporting mechanism for supporting a movable mirrorforming a part of an interferometric measurement system and having arectangular cross-section onto an object, with a reflecting surface ofthe movable mirror being arranged normal to a measuring direction, themovable mirror supporting mechanism including: at least two movablemirror supporting portions provided on the object and spaced apart fromeach other in a direction perpendicular to the measuring direction, eachof said movable mirror supporting portions having a top surface formedas a convex curved surface which gradually rises from the end edges tothe center thereof with respect to the measuring direction; and aplurality of fixtures for securing the movable mirror from above ontothe object at positions of the at least two movable mirror supportingportions.

With this movable mirror supporting mechanism, it is important that thetop surface of each movable mirror supporting surface is so formed as togradually rise from the end edges to the center thereof with respect tothe measuring direction. By virtue of this, when the movable mirror isfixedly secured from above onto the movable mirror supporting surfacesby the fixtures, any harmful deformation of the measuring surface (orthe reflecting surface) of the movable mirror may be effectivelyprevented even if the movable mirror itself is deformed by the stressproduced by the fixtures, because there is no portion of the movablemirror supporting surfaces which could prevent or interfere thedeformation of the movable mirror by the reaction force from it to themovable mirror.

According to a further aspect of the present invention, there isprovided a movable mirror supporting mechanism for supporting a movablemirror forming a part of an interferometric measurement system andhaving a rectangular cross-section onto an object, with a reflectingsurface of said movable mirror being arranged normal to a measuringdirection, the movable mirror supporting mechanism including: at leasttwo fixtures for securing the movable mirror from above onto respectivemovable mirror supporting portions provided on the object, each of saidfixtures having a rod portion and a large diameter portion at one end ofthe rod portion; and the large diameter portion of each fixture having aspherical convex surface facing to a top surface of the movable mirror,wherein the amount of projection of the spherical convex surface towardthe top surface of the movable mirror gradually increases from theperipheral portion of the spherical convex surface to the center of thestem.

With this movable mirror supporting mechanism, it is important that thelarge diameter portion of each fixture has a spherical convex surfacefacing to an upper surface of the movable mirror, and the amount ofprojection of the spherical convex surface toward the upper surface ofthe movable mirror gradually increases from the peripheral portion ofthe spherical convex surface to the center of the stem. By virtue ofthis, any nonuniform abutment of the large diameter portion of thefixture may be prevented even when the upper surface of the movablemirror to be secured by the fixture is inclined, so that any localstress concentration due to the nonuniform abutment may be prevented andthus any harmful deformation of the mirror surface of the movable mirrormay be prevented.

According to a still further aspect of the present invention, there isprovided a movable mirror supporting mechanism for supporting a movablemirror forming a part of an interferometric measurement system andhaving a rectangular cross-section onto a object, with a reflectingsurface of the movable mirror being arranged normal to a measuringdirection, the movable mirror supporting mechanism including: at leasttwo movable mirror supporting portions provided on the object and spacedapart from each other in the direction perpendicular to the measuringdirection, each of the movable mirror supporting portions having a topsurface for supporting the movable mirror; at least two fixtures forsecuring the movable mirror from above onto the movable mirrorsupporting portions provided on the object, each of the fixtures havinga rod portion and a large diameter portion at one end of the rodportion; attachment holes formed in the measuring object for receivingthe fixtures, each attachment hole having a shoulder associated with thelarge diameter portion of the fixture, the shoulder being inclined withrespect to an axis of the fixture; and a pair of ring-shapededge-abutment-preventing elements disposed in each attachment hole andinserted between the large diameter portion of the fixture and theshoulder of the attachment hole.

According to a yet further aspect of the present invention, there isprovided a stage including a movable stage body and a movable mirrorsecured onto the stage body, wherein: the movable mirror is made of amaterial consisting mainly of a highly compact ceramic material having arelatively low porosity; and the stage body is made of a materialconsisting mainly of another ceramic material having a higher porositythan the material for the movable mirror.

The ceramic material for the stage body may consist mainly of at leastone of ceramic materials selected from the group consisting of alumina(Al₂ O₃) ceramics, silicon nitride (Si₃ N₄) ceramics and sialon(Si--Al--O--N) ceramics. Further, the ceramic material for the movablemirror may be created by processing powder of the ceramic material forthe stage body through a hot-isostatic-pressing process.

With this stage, only the movable mirror may be made of a highly compactceramic material which is costly, so that the amount of the highlycompact ceramic material can be saved for the material piece support andthus the cost may be reduced over the case where both of the stage bodyand the movable mirror are made of a highly compact ceramic material.Further, since the stage body is made of a ceramic material, it may havea higher rigidity than a stage body made of any of glass materials.Moreover, since the movable mirror itself is made of a highly compactceramic material, it may enjoy a high reflectivity.

In addition, in the case where the ceramic material for the movablemirror is created by hot-isostatic-pressing the powder of the ceramicmaterial for the stage body, the ceramic material for the movable mirrorand that for the stage body will have the identical composition, so thatthe movable mirror and the stage body may have substantially the samethermal expansion coefficient. By virtue of this, any possibledeformation of the mirror surface of the movable mirror due to anenvironmental temperature change may be effectively suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be apparent from the following detailed description ofpreferred embodiments thereof, reference being made to the accompanyingdrawings, in which:

FIG. 1 a perspective view of a typical conventional movable mirrorsupporting mechanism;

FIG. 2 is a perspective view, partially broken away, of a movable mirrorsupporting mechanism according to a first embodiment of the presentinvention;

FIG. 3 is a perspective view, partially broken away, of a movable mirrorsupporting mechanism according to a second embodiment of the presentinvention;

FIG. 4 is an enlarged cross-sectional view of the movable mirrorsupporting mechanism shown in FIG. 3;

FIG. 5 is a perspective view, partially broken away, of a major portionof a movable mirror supporting mechanism according to a third embodimentof the present invention;

FIG. 6 is a cross-sectional view of a movable mirror illustrating thecondition of edge-abutment of the screw for securing the movable mirror;

FIG. 7 is a cross-sectional view of a major portion of a movable mirrorsupporting mechanism according to a fourth embodiment of the presentinvention;

FIG. 8 is a cross-sectional view of a major portion of a movable mirrorsupporting mechanism according to a first modification to the fourthembodiment of the present invention;

FIG. 9 is a cross-sectional view of a major portion of a movable mirrorsupporting mechanism according to a second modification to the fourthembodiment of the present invention;

FIG. 10 is a schematic representation showing a projection exposureapparatus using a stage according to an embodiment of the presentinvention;

FIG. 11 is a schematic frontal view of the stage of FIG. 10 for carryinga wafer (as viewed from the side opposite to that shown in FIG. 10);

FIG. 12 is a schematic side elevation of the stage of FIG. 10; and

FIG. 13 is a schematic plan view of the stage of FIG. 10.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the accompanying drawings, and in particular to FIGS. 2to 9 thereof, four movable mirror supporting mechanism according topreferred embodiments of the present invention will be described indetail.

FIG. 2 shows a movable mirror supporting mechanism according to a firstembodiment of the present invention. As shown, the movable mirrorsupporting mechanism comprises a stage body 12 which is an object whoseposition is to be measured. The stage body 12 has a belt-like raisedportion or attachment portion 14 having a fixed width and extendingalong one of four side edges of the stage body 12. A pair of movablemirror supporting portions or support protrusions 16A and 16B, which areof rectangular shape in plan view, are provided on the top surface ofthe attachment portion 14 at positions apart from each other in thelongitudinal direction of the attachment portion 14. The supportprotrusions 16A and 16B have respective bottom supporting surfaces 17Aand 17B formed as movable mirror supporting surfaces. A reflectingmirror or movable mirror 10 is supported on the bottom supportingsurfaces 17A and 17B so as to extend horizontally. The movable mirror 10has a pair of attachment holes 18A and 18B formed therein which extendvertically through the movable mirror 10 from the top surface to thebottom surface thereof. The attachment holes 18A and 18B receiverespective screws 30 (only one of the screws, which is received in thethrough hole 18A, is seen in FIG. 2), which have respective threadedends 30B in threading engagement with associated screw holes 34 formedin the bottom supporting surfaces 17A and 17B of the stage body 12 andthereby are secured to the stage body 12.

The movable mirror 10 has one side thereof (the right-hand side shown inFIG. 2) formed as a reflecting surface 20 or the measuring surface,which a laser beam emitted from a laser interferometer unit (not shown)is incident on and reflected by. The reflecting surface 20 is to bearranged normal to the measuring direction (represented by arrow A inFIG. 2). The movable mirror 10 comprises a glass body having arectangular cross-section and one side thereof silvered to form thereflecting surface 20.

The entire reflecting surface 20 of the movable mirror 10 shown in FIG.2 is considered to be divided into three areas including a usable area22A which may be utilized to reflect the laser beam from the laserinterferometer when the stage body 12 is moved to various positions in aplane and two unusable areas 22B which will never be used for themeasurement purpose although they are formed to be reflective. (Forclarity, the unusable areas 22B are represented by dotted regions inFIG. 2; however, the unusable areas 22B are actually silvered as withthe usable area 22A, and form a continuous reflecting surface with thelatter. Thus, in fact, there are not visible boundaries between theusable area 22A and the unusable areas 22B.) In this embodiment, theattachment holes 18A and 18B are formed at positions corresponding tothe unusable areas 22B of the reflecting surface 20, and thus themovable mirror 10 is fixedly secured onto the stage body 12 by thescrews 30 at these positions.

The stage body 12, here, forms a part of a wafer stage used in ademagnification projection exposure apparatus (or so-called "stepper")for the lithographic process in fabrication of semiconductor devices orthe like, in which the stepper serves to project an image of a circuitpattern formed on a mask (or reticle) through a projection lens onto aresist-coated photosensitized substrate (such as a wafer). The stepperperforms exposure operations to a plurality of exposure sites on a waferin a sequential manner while the stage body 12 carrying the wafer isstepped to these exposure sites, so that an inertial force acts on themovable mirror 10 due to the acceleration of the stage body 12 when thelatter is driven to start and stop its movement (i.e., when an exposureoperation is performed), and thus it is required to support the movablemirror 10 on the stage body 12 such that no displacement of the movablemirror 10 may occur relative to the stage body 12. The reflectingsurface 20 of the movable mirror 10 thus fixedly secured onto the stagebody 12 is used to reflect the laser beam from the laser interferometer(not shown) so as to measure the coordinate positions of the stage body12, which carries a wafer, with a vary high resolution (about 0.01 μm).

With the movable mirror supporting mechanism according to the firstembodiment of the present invention described above, the movable mirror10 is secured to the screw holes 34 formed in the bottom supportingsurfaces 17A and 17B on the stage body 12 by means of the two screws 30received in the two attachment holes 18A and 18B formed in and extendingthrough the movable mirror 10, so that the movable mirror 10 can befixedly secured onto the stage body 12. Accordingly, the movable mirror10 can never be subject to any displacement relative to the stage body12, which could otherwise occur due to an inertial force which may becaused by the stage body 12 being driven for stepping movement and acton the movable mirror 10, so that the position measurement using thelaser interferometer may be achieved with a very high resolution. Notethat the movable mirror is secured onto the stage body by the fixtures(the screws 30 in this embodiment) at two positions, or alternatively itmay be at more than two positions, so that any angular displacement ofthe movable mirror 10 relative to the stage body 12 can be effectivelyprevented as well.

Further, with the first embodiment described above, the movable mirror10 is secured by the screws 30 at the positions corresponding to theareas other than the usable area 22A of the reflecting surface 20 of themovable mirror 10 (e.g., to the unusable areas 22B of the reflectingsurface 20). Thus, any deformation of the reflecting surface 20, whichmay result from the deformation of the movable mirror 10 caused by thecompression forces acting on it from the tightened screws 30, will beconfined to the unusable areas 22B and not affect the usable area 22A atall.

FIG. 3 shows a movable mirror supporting mechanism according to a secondembodiment of the present invention. In FIG. 3, like elements aredesignated by the same reference numerals as used for the firstembodiment shown in FIG. 2 and will not be described in detail.

In the movable mirror supporting mechanism according to the secondembodiment, as with the first embodiment, a stage body 12 has anattachment portion 14, and a pair of movable mirror supporting portionsor support protrusions 32A and 32B are formed on the top surface of theattachment portion 14. The support protrusions 32A and 32B haverespective top surfaces formed as movable mirror supporting surfaces orbottom supporting surfaces 36A and 36B. Unlike the first embodiment,however, the bottom supporting surfaces 36A and 36B are formed asupwardly convex, curved surfaces. More specifically, each bottomsupporting surface 36A, 36B is formed as a convex curved surface whichgradually rises from the end edges to the center thereof with respect tothe measuring direction represented by arrow A in FIG. 3.

FIG. 4 is an enlarged cross-sectional view showing the movable mirror 10and the stage body 12 cross-sectioned along a vertical plane on whichthe axis of the screw 30 lies in order to illustrate the shape of thebottom supporting surface 36A more clearly. As shown in FIG. 4, thebottom supporting surface 36A (or the top surface of the supportprotrusion 32A), facing to the bottom of the movable mirror 10, is soshaped as to be a convex curved surface in which the height is lowest atpositions along both end edges in the measuring direction represented byarrow A, and in which the surface gradually rises from the end edges tothe center in the measuring direction at which the screw hole 34 isformed. This curved surface may comprise a cylindrical surface havingits axis extending parallel to and in the longitudinal direction of theattachment portion 14.

The arrangement of the movable mirror support mechanism shown in FIGS. 3and 4 will now be described in more detail. The movable mirror 10 has apair of screw head receiving attachment holes 18A and 18B at positionsat which the movable mirror 10 is secured onto the stage body 12 bymeans of the fixtures. Each attachment hole 18A, 18B has a circularcross-section and extends vertically from the top surface of the movablemirror 10 to a point whose height is about one half of the height of themovable mirror 10. In the bottom of the attachment hole 18A, a verticalthrough hole 19 is formed which is coaxial with and has a smallerdiameter than the attachment hole 18A. (FIG. 4 shows only one attachmenthole 18A; the other attachment hole 18B has the identical arrangementand hidden from view in FIG. 4.) The stage body 12, which supports themovable mirror 10, has a pair of movable mirror supporting portions orsupport protrusions 32A and 32B formed on the top surface of theattachment portion 14 which is formed along one side edge of the stagebody 12. A screw hole 34 having a female thread is formed in each of thesupport protrusions 32A and 32B at a position substantially at thecenter of that support protrusions in the measuring direction(represented by arrow A in FIG. 4). The female thread of the screw hole34 is formed for threading engagement with the male threaded portion 30Bof the fixture or screw 30. The fixture or screw 30 has a large diameterportion or head 30A and the rod or threaded portion 30B integral withand extending downwardly from the head 30A. The threaded portion 30Bhaving a smaller diameter than the head 30A.

The movable mirror 10 is placed on the bottom supporting surfaces 36Aand 36B of the stage body 12, with the screw holes 34 formed at thebottom supporting surfaces 36A and 36B being aligned with thecorresponding through holes 19 formed in the movable mirror 10. Then,the screws 30 are inserted in the corresponding attachment holes 18A and18B, the stems 30B of the screws 30 are brought into threadingengagement with the corresponding screw holes 34, and the screws 30 aretightened to complete the securing of the movable mirror 10 onto thestage body 12.

When the movable mirror 10 is secured onto the stage body 12 in thismanner, if the stage body 30 is made of a highly rigid metallic materialand the movable mirror 10 made of a glass material having a relativelylow rigidity is compressed against the stage body by the tightened screw30 at the position near the center of the movable mirror 10, the movablemirror 10 is subject to stress deformation which stretches vertically atboth end of the movable mirror 10 in the measureing direction (thedirection of arrow A). If the bottom supporting surfaces 32A and 32B areflat (prior art), then forces would act on such flat bottom supportsurfaces from the bottom of the movable mirror 10 deformed at both sidesas shown by arrows B in FIG. 4. Nevertheless, since the bottomsupporting surfaces made of a metallic material, would subject to almostno deformation, the reaction forces would act on the movable mirror 10from such bottom supporting surfaces (as represented by arrows C in FIG.4) to lead to some harmful deformation of the mirror surface 20 of themovable mirror 10.

In contrast, with the moving mirror supporting mechanism according tothe second embodiment of the present invention, because the bottomsupporting surfaces 36A and 36B are formed as convex curved surfaces(see FIG. 4), there are gaps defined between the edges of the bottomsupporting surfaces 36A and 36B and the associated edges of the bottomof the movable mirror 10. By virtue of the gaps, any possibledeformation of the movable mirror 10 will be free from any constraint,resulting in no reaction force acting from the bottom supporting surface36A to the movable mirror 10 (as represented by arrows C), and thus thereflecting surface 20 of the movable mirror 10 may be free from anyharmful deformation due to such reaction force.

Note that in FIG. 4 the curvature of the bottom supporting surface 36Aof the support protrusion 32A is shown with exaggeration for easierunderstanding. Actually, the movable mirror 10 will subject to only avery small deformation, and the bottom supporting surfaces 36A and 36Bhave a small curvature corresponding to such very small deformation.

FIG. 5 shows an arrangement of a movable mirror supporting mechanismaccording to a third embodiment of the present invention. In FIG. 5,like elements are designated by the same reference numerals as used inFIGS. 2 to 4 and will not be described in detail.

The third embodiment has a feature that, as shown in FIG. 5, a slipelement 46 is provided between i) the head 30A of fixture or screw 30and ii) the bottom or shoulder 38 of the attachment hole 18A, in orderto reduce any torque acting on the bottom 38 of the attachment hole fromthe head 30A of the screw 30.

The slip element 46 comprises a washer-like plate made of a suitablelow-friction synthetic resin, such as the resin produced by DuPont underthe trademark TEFLON. Of course, the slip element 46 is not limited tosuch a resin washer but may be formed in any of various other formswhich may reduce any frictional force acting between the screw 30 andthe bottom 38 of the hole 18A of the movable mirror 10. For example, theslip element 46 may comprise a metal washer with an appropriatelubricant applied on its surfaces, or may comprise a stack of slipperyfilms.

With this movable mirror supporting mechanism, as shown in FIG. 5, whenthe screw 30 is rotated right-handed (i.e., in the rotational directionshown by arrow D) to tighten the screw 30 so as to fix the movablemirror 10 onto the stage body 12, the head 30A of the screw 30 slidesagainst the top surface of the slip element 46 and/or the bottom surfaceof the slip element 46 slides against the bottom or shoulder 38 of theattachment hole 18A of the movable mirror 10, which may effectivelyreduce any frictional force acting between the head 30A and the bottomor shoulder 38 of the attachment hole 18A. Thus, the slip element 46 hasthe effect of reducing any frictional force acting between the head 30Aand the shoulder 38, with the result that any torque acting on theshoulder 38 of the attachment hole 18A of the movable mirror 10 from thescrew 30 being tightened may be substantially reduced.

Accordingly, by virtue of the arrangement in which the movable mirror 10is secured onto the stage body 12 with the slip element 46 insertedbetween each screw 30 and the movable mirror 10, any torque acting onthe movable mirror 10 from each screw 30 being tightened may besubstantially reduced, so that any twisting deformation of the movablemirror 10 which may be produced in the direction of arrow D due to suchtorque may be reduced, and thereby any harmful deformation of thereflecting surface 20 may be practically prevented.

FIG. 6 shows an arrangement in which an attachment hole 18A of a movablemirror 10 has a bottom or shoulder 38 which is not normal but inclinedwith respect to the axis of the attachment hole 18A and movable mirror10 is fixedly secured onto a stage body 12 by means of ordinary screws30. FIG. 7 shows an arrangement of a movable mirror supporting mechanismaccording to a fourth embodiment of the present invention. In FIGS. 6and 7, like elements are designated by the same reference numerals asused in FIGS. 2 to 5 and will not be described in detail.

The movable mirror supporting mechanism according to the fourthembodiment has a feature that, as shown in FIG. 7, a pair of ring-shapededge-abutment-preventing elements 47 and 48, facing to each other, areinserted between the head 30A of the screw 30 and the bottom or shoulder38 of the attachment hole 18A of the movable mirror 10. Theedge-abutment-preventing elements 47 and 48 serve to prevent nonuniformabutment of the head 30A of the screw 30 with the shoulder 38 of theattachment hole 18A even when the shoulder 38 is not normal but inclinedwith respect to the axis of the attachment hole 18A. Here, the"edge-abutment" of the screw head means the abutment of a partial edgeof the head 30A of the screw 30 with a partial area on the inclinedshoulder 38 of the attachment hole 18A, such as shown in FIG. 6, whichmay occur when the shoulder 38 is inclined and a screw 30 having a head30A of an ordinary shape (cylindrical or hexagonal) is used to fix themovable mirror 10 onto the stage body 12. The partial area on theshoulder 38 at which the partial edge of the head 30A of the screw 30 isin abutment with the shoulder 38 is subject to the concentration of thethrust force applied from the screw head 30A (the direction of thethrust force is represented by arrow E in FIG. 6), so that an unevenstress distribution will occur in the movable mirror 10 and that theconcentration of the stress may occur at a position adjacent to thereflecting surface 20 of the movable mirror 10. Under this situation,the effect of the deformation of the movable mirror 10 would have to betaken into consideration.

In view of the foregoing, the fourth embodiment comprises the pair ofring-shaped edge-abutment-preventing elements 47 and 48, in which oneelement 47 has a spherical concave top surface 47a and the other element48 has a spherical convex bottom surface 48a. The central holes of thering-shaped elements 47 and 48 receive the rod or threaded portion 30Bof the screw 30. The radius of curvature of the spherical concave bottomsurface 47a is preferably the same as or slightly greater than that ofthe spherical convex top surface 48a.

Accordingly, the spherical convex surface 48a extends just under thehead 30A of the screw 30 and around the stem 30B of the screw 30.Further, the amount of projection of the spherical convex surface 48atoward the bottom 38 of the attachment hole 18A gradually increases fromthe peripheral portion of the surface 48a to the stem 30B of the screw30. On the other hand, the spherical concave surface 47a, which faces tothe spherical convex surface 48a, extends around the stem 30B of thescrew 30, with the depth of the spherical concave surface 47a increasingfrom the peripheral portion of the surface 47a to the stem 30B of thescrew 30 extending at the center of the surface 47a. When the screw 30is tightened, the spherical convex surface 48a and the spherical concavesurface 47a are brought into abutment with each other. If the bottom orshoulder 38 of the attachment hole 18A is not normal to the axis of thescrew 30, then the abutment point between the spherical surfaces 48a and47a will be automatically adjusted, so that any edge-abutment of thescrew head 30A may be effectively prevented and the thrust force appliedby the screw head 30A may be distributed over a wide area on the bottomor shoulder 38 of the attachment hole 18A so as to prevent any stressconcentration thereon.

The movable mirror supporting mechanism according to the fourthembodiment of the present invention comprises not only the arrangementfor preventing edge-abutment just described, but also all the featurespreviously described with reference to the movable mirror supportingmechanisms according to the first, second and third embodiments of thepresent invention.

More specifically, the movable mirror 10 shown in FIG. 7 is fixedlysecured onto the stage body 12 by means of two screws (only one screw 30is shown in FIG. 7; the other screw is hidden from view) tightened,which are in threading engagement with corresponding two screw holes 34formed in associated two bottom supporting surfaces (only one bottomsupporting surface 36A is shown in FIG. 7; the other bottom supportingsurface 36B is hidden from view), respectively, of the stage body 12.Although not shown in FIG. 7, those positions at which the movablemirror 10 is fixedly secured by the screws 30 are defined according tothe same criterion as used in the first embodiment shown in FIG. 2,i.e., such positions are defined to correspond to the areas other thanthe usable area 22A of the reflecting surface 20 (e.g., to the unusableareas 22B of the reflecting surface 20).

Further, in the fourth embodiment, as with the second embodiment shownin FIGS. 3 and 4, the stage body 12 has a projecting attachment portion14 formed on one end the stage body 12 and a pair of movable mirrorsupporting portions or support protrusions 32A (see FIG. 3 for the othersupport protrusion 32B) are provided on the top surface of theattachment portion 14 at positions apart from each other for supportingthe movable mirror 10 in the vertical direction. The support protrusion32A have respective bottom supporting surfaces 36A (see FIG. 3 for theother supporting surface 36B) of cylindrical shape. The movable mirror10 is supported on the bottom supporting surface 36A. Each of the bottomsupporting surfaces is formed as a convex curved surface which graduallyrises from the end edges to the center thereof with respect to themeasuring direction (represented by arrow A) of the interferometricmeasurement system.

Moreover, in the fourth embodiment, as with the third embodiment shownin FIG. 5, a slip element 46 is inserted between the bottom 38 of theattachment hole 18A formed in the movable mirror 10 shown in FIG. 7 andthe bottom of the lower edge-abutment-preventing element 47 in order toreduce any frictional force acting between them.

As understood from the above, the movable mirror supporting mechanismaccording to the fourth embodiment has not only the features specific toitself but also those features which are seen in the first, second andthird embodiments described above, so that the fourth embodiment canprovide the combination of all the desirable effects which may beobtained by virtue of such features.

More specifically, with the movable mirror supporting mechanismaccording to the fourth embodiment shown in FIG. 7, the movable mirror10 is secured onto the stage body 12 by means of two screws 30, so thatany displacement of the movable mirror 10 relative to the stage body 12may be prevented with reliability; such displacement could otherwise becaused by the inertial force acting to the movable mirror 10 due to itsacceleration occurring when the stage body 12 is driven to start of stopmoving. In addition, with the fourth embodiment, the movable mirror 10is fixed by the screws 30 at the positions corresponding to the areasother than the usable area 22A of the reflecting surface 20 of themovable mirror 10 (see FIG. 1), so that any deformation of thereflecting surface 20, which may result from the deformation of themovable mirror 10 caused by the compression forces acting on it from thetightened screws 30, will be confined to the unusable areas 22B and notaffect the usable area 22A at all.

Further, with the fourth embodiment, because of the tightening of thescrews 30 in order to fixedly secure the movable mirror 10 onto thestage body 12, the movable mirror 10 may subject to such deformationthat tends to stretch in the vertical direction at both sides of themovable mirror 10 in the measuring direction (represented by arrow A).Nevertheless, since the bottom supporting surfaces 36A and 36B areformed as convex curved surfaces and thus are out of contact with thebottom of the mirror 10 at regions along both sides of the movablemirror 10, such deformation of the movable mirror 10 is not suppressedby the bottom supporting surfaces 36A and 36B at all, resulting in thatno secondary stress will occur in the movable mirror 10 due to thereaction forces which could otherwise be applied to the movable mirror10 from the bottom supporting surfaces 36A and 36B, and thereby anyharmful deformation of the mirror surface 20 may be effectivelyprevented.

Moreover, with the fourth embodiment, the slip element 46 made of asuitable material such as a synthetic resin is inserted between thebottom 38 of the attachment hole 18A formed in the movable mirror 10 andthe bottom of the lower edge-abutment-preventing element 47 so as toreduce any frictional force acting between them. By virtue of this, anyrotational moment (or torque) acting on the bottom 38 of the attachmenthole 18A from the screw 30 being tightened for fixedly securing themirror 10 to the bottom supporting surface 36A of the stage body 12, maybe substantially reduced. This reduction in the rotational momentprovides the effect of reducing the stress which may be induced by therotational moment, so that any harmful deformation of the reflectingsurface 20 may be effectively prevented.

In addition, as shown in FIG. 7, between the slip element 46 and thehead 30A of the screw 30, there are provided theedge-abutment-preventing element 47 having the spherical concave surface47a and the edge-abutment-preventing element 48 having the sphericalconvex surface 48a, with the spherical surfaces 47a and 48a facing toeach other. By virtue of this, even when the shoulder 38 is inclinedand/or the axis of the screw hole 34 is inclined with respect to theassociated bottom supporting surface 36A, 36B so that the screw 30 inengagement with the screw hole is inclined, the point of abutmentbetween the spherical concave surface 47a of one element 47 and thespherical convex surface 48a of the other element 48 may beautomatically adjusted depending on the angles of these tilts so as toprevent any edge-abutment, and thereby any harmful deformation of thereflecting surface 20 of the movable mirror 10 may be effectivelyprevented.

Now, modifications to the fourth embodiment above will be described.FIG. 8 shows a movable mirror supporting mechanism in which a simplerarrangement is used to prevent the edge-abutment which could otherwiseoccur due to the inclination of the shoulder 38 of the attachment hole18A of the movable mirror 10. More specifically, this movable mirrorsupporting mechanism has a feature that the movable mirror 10 is securedby means of such screws 40 in which each screw 40 has a head 40A whosebottom surface 42, facing the shoulder 38 of the attachment hole 18A ofthe movable mirror 10, is formed as a spherical convex surfaceprojecting toward the shoulder 38 and the projection amount of thespherical convex surface toward the shoulder 38 of the attachment hole18A gradually increases from the peripheral portion of the sphericalconvex surface to the center of a stem 40B of the screw 40.

By virtue of the spherical convex shape of the bottom surface 42 of thehead 40A of the screw 40, any edge-abutment of the head 40A against theshoulder 38 of the attachment hole 18A of the movable mirror 10 may beavoided even when the shoulder 38 is at a tilt with respect to the axisof the attachment hole 18A, because when the screw 40 is tightened theboundary portion of the screw 40 between its head 40A and stem 40Balways come into abutment with the inner edge portion of the shoulder 38near the screw hole 34. As a result, any harmful deformation of themirror surface 20 of the movable mirror 10 may be effectively prevented.

FIG. 9 shows another modification to the fourth embodiment. Themodification of FIG. 9 is identical to that of FIG. 8 above except thatthe shoulder 38 of the attachment hole 18A is formed as a sphericalconcave surface. With this modification, any partial or localizedabutment of the edge of the screw head 40A against the shoulder 38 maybe effectively prevented as well.

As understood from the above, with each and any of the movable mirrorsupporting mechanisms according to the first to fourth embodiments ofthe present invention described above, i) the movable mirror is securedonto the stage body so that any displacement of the former relative tothe latter may be effectively prevented, ii) any deformation of themovable mirror which may be produced by the compression forces appliedfrom the fixtures of screws being tightened may be minimized and iii)any harmful deformation of the reflecting surface of the movable mirrorwhich may be produced by possible deformation of the movable mirror maybe minimized. By virtue of these, the position measurement by the laserinterferometric measurement system may be achieved with very highresolution.

In each of the embodiments described above, the movable mirror isfixedly secured onto the stage body by means of two fixtures or screws.Alternatively, three or more fixtures may be used for this purpose. Insuch cases, it is preferable that the positions at which the movablemirror is secured by such fixtures be confined to areas other than theusable area where the reflecting surface of the movable mirror isutilized for interferometric measurement.

Further, in each of the embodiments described above, the fixturescomprise screws. Alternatively, the fixtures may comprise any othersuitable means including rivets.

In some of the embodiments described above, a slip element is insertedbetween the screw and the movable mirror in order to reduce any torqueacting on the movable mirror from the screw when the latter is tightenedfor securing the movable mirror onto the stage body. Alternatively,instead of using such a slip element, at least one of the surfaces ofthe screw and of the movable mirror contacting with each other may becoated with a layer of a suitable low-friction material such as theresin produced by DuPont under the trademark TEFLON or may have asuitable lubricant applied.

In the fourth embodiment described above, as shown in FIG. 7, theelements having the spherical concave and convex surfaces 47a and 48a,respectively, are inserted, together with the slip element 46, betweenthe screw and the movable mirror. As one possible modification to this,the elements 47 and 48 may be made of a suitable low-friction materialsuch as a certain synthetic resin so as to eliminate the slip element46.

In the modifications to the fourth embodiment described above, thebottom surface 42 of the head 40A of the screw 40 is formed as aspherical convex surface. Alternatively, the bottom surface 42 may beformed to have any of various other suitable shapes, includingcontiguous curved surfaces and conical surfaces.

Each of the embodiments described above shows an exemplified applicationof the present invention to an exposure apparatus for thestep-and-repeat exposure operation. The applications of the presentinvention, however, are not limited to such as exposure apparatus. Infact, the present invention may be also applicable to various othertypes of exposure apparatus including those for the step-and-scanexposure operation. Further, the present invention may be convenientlyand desirably applicable to many devices and apparatus using aninterferometric measurement system for the purpose of positionmeasurement.

As is clearly understandable from the above, according to the movablemirror supporting mechanisms of the above described embodiments, themovable mirror is secured onto the object to be measured so that anydisplacement of the movable mirror relative to the object may beeffectively prevented, while any harmful deformation of the reflectingsurface, which may possibly be caused by the deformation of the movablemirror itself resulting from the stress induced in the movable mirrorwhen it is fixed onto the stage body, may be effectively minimized.

Now, a stage according to a preferred embodiment of the presentinvention will be described. The stage is a wafer stage used in aprojection exposure apparatus for use in fabrication of semiconductorintegrated circuit chips.

FIG. 10 shows the projection exposure apparatus 100 using the waferstage according to the present invention. The projection exposureapparatus 100 is a scanning projection exposure apparatus, in which areticle R and a wafer W are scanned in synchronism with each other andrelative to the exposure light beam and a pattern formed on the reticleR is sequentially transferred onto a plurality of exposure sites on thewafer W. The projection exposure apparatus 100 is a so-called "scanningstepper".

As shown in FIG. 10, the reticle R is vacuum-chucked onto a reticlestage 120 and an illumination optical system 101 including a lightsource emits an exposure light beam so as to illuminate the patternbearing surface of the reticle R with a substantially uniformilluminance distribution. The reticle stage 120 is capable oftranslational and rotational movements in an XY-plane by means of areticle drive unit 103, where the XY-plane is defined as a planeperpendicular to the Z-axis which is coincident with the optical axis ofa projection optical system 102. The reticle stage 120 has reflectingmirrors or movable mirrors 126 and 127 fixedly secured thereon forinterferometric position measurement. One movable mirror 126 is usedtogether with an interferometer 104 so as to measure a position in theX-direction (i.e., the left and right direction on the sheet surface) ofthe reticle stage 120. More specifically, the interferometer 104 emits alaser beam to the movable mirror 126, and the reflected beam from themovable mirror 126 is used for measuring the position of the reticlestage 120 in the X-direction. Similarly, the other movable mirror 127 isused together with a second interferometer (not shown) so as to measurethe position of the reticle stage 120 in the horizontal directionperpendicular to the sheet surface.

With the exposure light beam being illuminated to the pattern formed onthe reticle R, the pattern is transferred through the projection lens102 onto the wafer W which is vacuum-chucked onto the wafer stage 130 (awafer holder 134 of the wafer stage 130).

The wafer stage 130 has reflecting mirrors or movable mirrors 136 and137 fixedly secured thereon for interferometric position measurement.One movable mirror 136 is used together with an interferometer 106 so asto measure the position of the wafer stage 130 in the X-direction. Morespecifically, the interferometer 106 emits laser beams to the movablemirror 136 and to a fixed mirror (not shown) provided for the projectionoptical system 102, respectively, and the interference between thereflected beams from the fixed mirror and from the movable mirror 136,respectively, is used for measuring the position of the wafer stage 130in the X-direction. Similarly, the other movable mirror 137 is usedtogether with another interferometer (not shown) so as to measure theposition of the wafer stage 130 in the Y-direction.

The wafer stage 130 is capable of three-dimensional translationalmovement, i.e., translational movement in X-, Y- and Z-directions, bymeans of a wafer drive unit 105. An alignment microscope 108 is disposedon a side of the projection lens 102. The alignment microscope 108 is ofthe off-axis type, in which the optical axis of the microscope is notcoincident with that of the projection lens 102. The alignmentmicroscope 108 is used to detect a predetermined alignment marks (notshown) from a plurality of areas on the wafer W. The positions of thealignment marks thus detected are processed using the knownleast-square-approximation method so as to abtain the position (orcoordinates) of the wafer W. Further, the baseline amount, which is thedistance between the optical axis of the alignment microscope 108 andthat of the projection lens 102, is determined and used to move ordisplace the wafer stage 130 by that amount when the exposure operationsequence for the wafer W starts.

FIG. 11 shows a frontal view of the wafer stage 130 (as viewed from theside opposite to that shown in FIG. 10), and FIG. 12 shows a sideelevation of the wafer stage 130 (as viewed from the right-hand side inFIG. 10). FIGS. 11 and 12 do not show the movable mirror 137 shown inFIG. 10. Further, the movable mirror 137 will not be described in detailsince it has the arrangement similar to that of the movable mirror 136.FIG. 13 illustrates the position of the wafer W relative to the movablemirrors 136 and 137 on the wafer stage 130.

In FIGS. 11 and 12, the stage body 133 is capable of movement in anXY-plane by means of an XY-stage 131. The stage body 133 is also capableof movement in the Z-direction and the vertical direction by means of aZ-stage 132. The wafer W is placed on the stage body 133 through a waferholder 134.

The movable mirror 136 is secured directly to the stage body 133 by apair of set screws 135a and 135b. More specifically, the set screws 135aand 135b are inserted from underside of the stage body 133 intorespective holes formed in the stage body 133, and brought intothreading engagement with respective screw holes formed in the bottom ofthe movable mirror 136. By virtue of this arrangement, the strength ofthe connection between the movable mirror 136 and the stage body 133 isimproved over any of various conventional connections between them, sothat any possible displacement of the movable mirror 136 relative to thestage body 133, which may occur due to acceleration or deceleration ofthe stage body 133 when it is moved, can be effectively minimized. Themovable mirror 136 has a pair of slits 140a and 140b formed therein atpositions just above the positions corresponding to the set screws 135aand 135b. The slits 140a and 140b extend through the movable mirror 136in the direction of a laser beam B1 emitted from the associatedinterferometer unit 106. In this manner, the slits 140a and 140b areformed between a portion of the movable mirror 136 which bears thereflecitng surface 141, and respective portions of the movable mirror136 at which the movable mirror 136 is secured onto the stage body 133by the set screws 135a and 135b. This results in that any deformation ofthe movable mirror 136, which may be caused by the set screws 135a and136b being or having been tightened in order to secure the movablemirror 136 onto the stage body 133, will not propagate to the reflectingsurface 141 of the movable mirror 136. The number and the size of theslits formed for this purpose may be modified depending on relevantfactors, including the size of the movable mirror to be secured, forexample.

As shown in FIG. 13, like the reflecting mirror or movable mirror 136described above, the other reflecting mirror or movable mirror 137 isalso secured directly to the stage body 133 by a pair of set screws 138aand 138b. By virtue of this arrangement, the strength of the connectionbetween the movable mirror 137 and the stage body 133 is improved overany of various conventional connections between them, so that anypossible displacement of the movable mirror 137 relative to the stagebody 133, which may occur due to acceleration or deceleration of thestage body 133 when it is moved, can be effectively minimized. Themovable mirror 137 has a pair of slits 142a and 142b formed therein atpositions just above the positions corresponding to the set screws 138aand 138b. The slits 142a and 142b extend through the movable mirror 137in the direction of a laser beam B2 emitted from the associatedinterferometer unit (not shown). In this manner, the slits 142a and 142bare formed between a portion of the movable mirror 137 which bears thereflecting surface 143 and respective portions of the movable mirror 137at which the movable mirror 137 is secured onto the stage body 133 bythe set screws 138a and 138b. As with the movable mirror 136 describedabove, this results in that any deformation of the movable mirror 137,which may be caused by the set screws 138a and 138b being or having beentightened in order to secure the movable mirror 137 onto the stage body133, will not propagate to the reflecting surface 143 of the movablemirror 137.

As shown in FIG. 13, the position of the set screws 135a and 135b on themovable mirror 136 are defined to be out of the maximum usable area D1of the movable mirror 136. More specifically, the positions of the setscrews 135a and 135b on the movable mirror 136 are defined to be out ofthat area of the reflecting surface 141 which is taken in thelongitudinal direction of the movable mirror 136 (or the Y-direction)and within which the laser beam B1 can be incident on the mirror surface141. The positions are defined in this manner since any deformation ofthe movable mirror 136 which may be produced when the set screws 135aand 135b are tightened in order to secure the movable mirror 136 ontothe stage body 133 will have maxima at the positions of the set screws135a and 135b. Thus, by defining the positions of the set screws 135aand 135b as shown in FIG. 13, any harmful deformation of the reflectingsurface 141 of the movable mirror 136 may be minimized, resulting inthat any possible error in the measured values produced from theinterferometer unit 106 due to such deformation may be minimized. It isadvantageous that the positions of the set screws 135a and 135b bedefined to be distant from the usable area of the movable mirror 136 asfar as possible.

Regarding to the other movable mirror 137, as with the movable mirror136 described above, the positions of the set screws 138a and 138b aredefined to be out of the maximum usable area D2 of the movable mirror137. By virtue of this, any harmful deformation of the reflectingsurface 143 of the movable mirror 137, which may be possibly producedwhen the set screws 138a and 138b are tightened, may be effectivelyminimized.

In operation, the stage body or material piece support 133 is driven bymeans of the XY-stage 131 and the Z-stage 132 for desirable movements,while the position of the stage body 133 (and thus of the wafer W) ismeasured by using the interferometer 106 (see FIG. 10). Morespecifically, the laser beam B1 emitted to and incident on the movablemirror 136 is reflected by it and received by the interferometer 106,which measures the displacement of the stage body 133 (and thus of thewafer W) relative to the optical axis of the projection lens 102. Inthis measurement the position of the stage body 133 can be measured withprecision because the movable mirror 136 is secured directly to thestage body 133 by the set screws 135a and 135b so that there can bealmost no displacement of the movable mirror 136 relative to the stagebody 133. When it is required to detach the movable mirror 136 from thestage body 133 for some reason or other, i.e., for repolishing thereflecting surface 141 of the movable mirror 136 or for repairing themovable mirror 136, such detachment may be easily achieved by looseningthe set screws 135a and 135b. These merits are enjoyed by the othermovable mirror 137 as well.

The material of the stage body 133 may consist mainly of any suitable,relatively inexpensive ceramic material having relatively high porosity.For example, the stage body 133 may be made of a ceramic materialconsists mainly of any of alumina (Al₂ O₃) ceramics, silicon nitride(Si₃ N₄) ceramics and sialon (Si--Al--O--N) ceramics. On the other hand,the material of the movable mirrors 136 and 137 which are required tohave high reflectivity may consist mainly of any suitable, highlycompact ceramic material having relatively low porosity. For example,the movable mirrors 136 and 137 may be made of a highly compact ceramicmaterial which is created by processing the powder of the ceramicmaterial for the stage body 133 through a known hot-isostatic-pressingprocess so as to improve compactness. This results in substantially thesame thermal expansion coefficient between the stage body 133 and themovable mirrors 136 and 137, so that any possible deformation of thereflecting surfaces 141 and 134 of the movable mirrors 136 and 137,respectively, which may be caused by an environmental temperaturechange, may be effectively suppressed.

As described, in this embodiment, only the reflecting mirrors or movablemirrors 136 and 137 are made of a highly compact ceramic material (whichis costly), so that the amount of the highly compact ceramic materialcan be saved for the stage body 133 so as to achieve a considerablereduction in the required amount of such a costly material, and thus thecost may be reduced over the case where all of the stage body 133 andthe movable mirrors 136 and 137 are made of a highly compact ceramicmaterial. Further, since the stage body 133 is made of a ceramicmaterial, it may have a higher rigidity than a stage body made of any ofglass materials. Moreover, since the movable mirrors 136 and 137themselves are made of a highly compact ceramic material, they may enjoya high reflectivity.

In addition, in the case where the ceramic material for the movablemirrors 136 and 137 is created by hot-isostatic-pressing the powder ofthe ceramic material for the stage body 133, the ceramic material forthe movable mirrors 136 and 137 and that for the stage body 133 willhave the identical composition, so that the movable mirrors and thestage body may have substantially the same thermal expansioncoefficient. By virtue of this, any possible deformation of thereflecting surfaces 141 and 143 of the movable mirrors 136 and 137 dueto an environmental temperature change may be effectively suppressed.

Having described the present invention with reference to severalpreferred embodiments thereof, it is to be understood that the presentinvention is not limited to the preferred embodiments described, but maybe embodied in various other forms and arrangements without departingthe spirit and the scope of the present invention. For example, althougheach of the embodiments described shows an exemplified application ofthe present invention to a wafer stage in an exposure apparatus, thepresent invention is also applicable to a reticle stage in an exposureapparatus. Further, the present invention may be applicable to variousother types of stages using an interferometric measurement system, suchas a pattern position measuring system for measuring the coordinates ofa pattern formed on a reticle.

The present patent application is based on Japanese Patent ApplicationsNo. 8-115332 filed on Apr. 12, 1996 and No. 8-221753 filed on Aug. 5,1996. The entire disclosure of each of the Japanese patent application,including its specification, claims, drawings and summary, isincorporated herein by reference in its entirety.

What is claimed is:
 1. A stage device comprising:a body; a movablemirror used for interferometric measurement of the position of said bodyand having a rectangular cross-section and a reflecting surface beingarranged normal to a measuring direction of said body; and a supportingmechanism which supports said movable mirror, and includes at least twofixtures for securing said movable mirror from a top side perpendicularto said reflecting surface onto respective movable mirror supportingportions provided on said body, and portions on said movable mirror atwhich said movable mirror is secured onto said body by said fixturesbeing defined to be out of that area of said reflecting surface of saidmovable mirror which is utilized for interferometric measurement.
 2. Astage device according to claim 1, wherein said fixture comprises ascrew.
 3. A stage device according to claim 2, wherein a slip element isinserted between said screw and said movable mirror for reducing anytorque acting on said movable mirror from said screw when the latter isbeing tightened.
 4. A stage device comprising:a body; a movable mirrorused for interferometric measurement of the position of said body, andhaving a rectangular cross-section and a reflecting surface beingarranged normal to a measuring direction of said body; and a supportingmechanism which supports said movable mirror, and includes at least twosupporting portions provided on said body and spaced apart from eachother in a direction perpendicular to said measuring direction, and aplurality of fixtures for securing said movable mirror from above ontosaid body at positions of said at least two movable mirror supportingportions; wherein each of said supporting portions has a top surfaceformed as a convex curved surface which has a first edge and a secondedge opposite to the first edge and a center portion between said firstedge and said second edge with respect to said measuring direction, andthe convex curved surface gradually rises from said first edge to saidcenter portion and gradually descends from said center portion to saidsecond edge.
 5. A stage device according to claim 4, wherein saidfixture comprises a screw.
 6. A stage device according to claim 5,wherein a slip element is inserted between said screw and said movablemirror for reducing any torque acting on said movable mirror from saidscrew when the latter is being tightened.
 7. A stage device comprising:abody; a movable mirror used for interferometric measurement of theposition of said body and having a rectangular cross-section and areflecting surface being arranged normal to a measuring direction ofsaid body; and a supporting mechanism which supports said movablemirror, and includes at least two fixtures for securing said movablemirror from above onto respective movable mirror supporting portionsprovided on said body, each of said fixtures having a rod portion and alarge diameter portion connected at one end of said rod portion; andsaid large diameter portion of each fixture including a projectionhaving a spherical convex surface facing an upper surface of saidmovable mirror, wherein the amount of projection of said sphericalconvex surface gradually increases from a peripheral portion of saidlarge diameter portion to a connecting portion between said rod portionand said large diameter portion.
 8. A stage device according to claim 7,wherein said fixture comprises a screw.
 9. A stage device according toclaim 8, wherein a slip element is inserted between said screw and saidmovable mirror for reducing any torque acting on said movable mirrorfrom said screw when the latter is being tightened.
 10. A stage devicecomprising:a body; a movable mirror used for interferometric measurementof the position of said body, and having a rectangular cross-section anda reflecting surface being arranged normal to a measuring direction ofsaid body; and a supporting mechanism which supports said movablemirror, and includes: at least two supporting portions provided on saidbody and spaced apart from each other in the direction perpendicular tosaid measuring direction, each of said supporting portions having a topsurface for supporting said movable mirror; at least two fixtures forsecuring said movable mirror from above onto said supporting portions,each of said fixtures having a rod portion and a large diameter portionconnected at one end of said rod portion; attachment holes formed insaid movable mirror for receiving said fixtures, each of said attachmentholes having a shoulder associated with said large diameter portion ofsaid fixture, said shoulder being inclined with respect to an axis ofsaid fixture; and a pair of ring-shaped edge-abutment-preventingelements disposed in each attachment hole and inserted between saidlarge diameter portion of said fixture and said shoulder of saidattachment hole.
 11. A stage device according to claim 10, wherein saidfixture comprises a screw.
 12. A stage device according to claim 11,wherein said pair of ring-shaped edge-abutment-preventing elementsincludes a first element disposed adjacent to said large diameterportion and having a bottom surface formed as a spherical convex surfaceand a second element disposed on said shoulder and having a top surfaceformed as a spherical concave surface in engagement with said sphericalconvex surface.
 13. A stage device according to claim 12, wherein a slipelement is inserted between one of said edge-abutment-preventingelements and said shoulder of said movable mirror for reducing anytorque acting on said movable mirror from said screw when the latter isbeing tightened.